1. Field of the Invention
The present invention relates generally to microwave amplifiers and, more particularly, relates to a microwave amplifier utilizing a feed-forward technique.
2. Brief Description of the Relevant Art
Feed-forward is a standard distortion cancellation technique which is very effective for removing distortion from an amplified signal. When a microwave input signal is amplified the amplitude of the input signal is increased by a factor equal to the gain of the amplifier. The output signal of the amplifier includes a component representing the amplified input signal and a distortion component that is not a true representation of the input signal.
In a feed-forward system this distortion component is sampled, amplitude matched to the distortion component of the amplifier output signal, and recombined 180.degree. out of phase with the amplitude output signal to cancel the distortion component in the amplitude output signal.
Complete cancellation requires that the sampled distortion signal be exactly 180.degree. out of phase with the distortion component of the amplifier output signal. In the feed-forward system this phase relationship is achieved by introducing fixed delay into the signals. Thus, the attainment of the required phase relationship depends on the accuracy of magnitudes of the introduced delays.
The phase error caused by a delay error of a fixed magnitude increases as the frequency of the signal increases. Thus, accuracy limitations imposed by the physical embodiment of a delay line and amplifier elements have limited the use of the feed-forward technique to applications below some threshold frequency.
Additionally, delay matching requires that the delay introduced by the amplifier components of the system be of fixed amplitude. However, for signals in the gigahertz (GHz) range amplifiers exhibit phase dispersion and the delay introduced into each frequency component of the amplifier input signal is not constant.
Thus, the required phase relationship may not be obtained for different frequency components of a broadband input signal regardless of the accuracy obtainable by the circuit delay elements.
Accordingly, the utilization of the feed-forward technique has not been possible for broadband input signals in the GHz range.